1. Field of the Invention
The present invention relates to an n-type silicon wafer having a &lt;111&gt; crystal axis suitable for a zener diode and the like, and to a method for producing such a silicon wafer.
2. Discussion of the Background
A zener diode in general has a structure in which an n-type silicon substrate is selectively doped with a p-type impurity. As the n-type silicon substrate, a silicon wafer doped with a group V element, such as phosphorus, is used so that its specific resistance is as low as several milliohms to several ohms. As the silicon wafer, a wafer having a &lt;111&gt; crystal axis is preferred from the viewpoint of electrical properties, in particular its operating resistance, which is normally produced through a growing process by the Czochralski method.
Since the resistivity distribution in the surface of an n-type wafer having a &lt;111&gt; crystal axis is uneven, the use of n-type wafers having a &lt;100&gt; crystal axis or a &lt;511&gt; crystal axis is also considered in some fields (e.g. Japanese Patent Application Laid-Open No. 4-266065).
However, the electrical properties of n-type wafers having &lt;100&gt; or &lt;511&gt; crystal axes are typically inferior to n-type wafers having a &lt;111&gt; crystal axis. Specifically, n-type wafers having &lt;100&gt; or &lt;511&gt; crystal axes can be used only within a certain operating voltage section. Due to this problem, n-type wafers having a &lt;111&gt; crystal axis are still desired as n-type wafers for zener diodes. However, n-type wafers having a &lt;111&gt; crystal axis have the following problems related to the properties of zener diodes.
One of the properties of zener diodes is that their operating voltage sections are divided into very narrow sections. When used as the material for zener diodes, therefore, silicon wafers are required to have an even resistivity distribution in the wafer surface. However, n-type wafers having a &lt;111&gt; crystal axis have a problem that the evenness of the resistivity distribution is essentially poor. Therefore, the yield of good products in the wafers is considerably low.
In order to solve such a problem, it is effective to some extent to reduce the rotation rate of the crucible in the process for growing a single crystal rod, which is the material for silicon wafers, to make the doping element distribution even in the direction of the crystal diameter. In fact, the rotation rate is considerably lower than the rotation rate used in ordinary growing by the Czochralski method. When the rotation rate of the crucible is reduced, however, the convection of the molten silicon tends to transport foreign substances toward the crystal, accelerating the dislocation. Therefore, the yield of single crystals is lowered, and, from this point of view, the rotation rate cannot be greatly reduced.
The uneven distribution of the doping element in a single crystal is more significant as the crystal diameter increases. In order to prevent uneven distribution, further reduction of the rotation rate of the crucible is required, but this accelerates the dislocation.
For these reasons, the resistivity distribution in the surface of an n-type wafer having a &lt;111&gt; crystal axis, as represented by .DELTA..rho.={(.rho.max-.rho.min)/.rho.min)}/.times.100, cannot be kept at 10% or less. In addition, the crystal diameter is limited to 4 inches or less.
The above description is not limited to wafers produced by the Czochralski method;
the same problems exist in wafers produced by the floating zone melting (FZ) method.
In order to solve these problems in n-type wafers having a &lt;111&gt; crystal axis, a method is known to radiate neutrons in a nuclear reactor to convert a part of an isomer of silicon into phosphorus. However, without doping, this method is expensive. Thus, there still exists a need for providing a practical and inexpensive n-type water having a &lt;111&gt;crystal axis having an even resistivity distribution in its surface.